The recent development of the NFL (natural laminar flow) and LFC (laminar flow control)airfoils for the reduction of friction drag heavily relies on our knowledge of the mechanism of laminar-turbulent transition. In this report, examined are the following problems closely related to the prediction and control of boundary layer transition.First, the receptivity, the process by which external disturbances are internalized as T-S waves, is investigated through direct Navier-Stokes simulations to understand the necessary condition for the occurrence of instability waves(T-S waves) and the coupling coefficient. A nonlinear receptivity process generating T-S waves is also examined through laboratory and numerical experiments. The results show that the external disturbance of frequency f can generate T-S wave of 2f (as well as decayed T-S wave of f) if the frequency f is much below the lower branch of the neutral stability curve.Second, we examined nonlinear response of boundary layer to high-intensity vortices at subcritical Reynolds numbers experimentally and numerically. Our knowledge of such nonlinear response is crucial for understanding the attachment-line contamination on a swept wing. The results show that when highly disturbed by energetic hairpin eddies convecting from the leading edge, the near wall flow develops hairpin eddies in succession to lead to the subcritical transition beyond the x-Reynolds number R_x=3.8x10^4. We also examined the effect of riblets on the development of wall turbulence structure in this transitional flow.